In small spaces, you will want many airchanges per hour worth of ventilation to avoid buildup of co2 and other human and building material gases. To do this and maintain a desired temperature range inside the van, you'll want a heat exchanger. if your intake pipe,is 3in duct run inside a 4in duct exhaust line for the length of the van, that would do.Ts to split the inner and outer ducts and then put your intakes and exhaust at oppsite corners. both inside the van and outside it. Keep your fresh air intake as far from the engine exhaust pipe as possible and have a good Carbon monoxide alarm.
You are a hot moist ball of meat, the van is a big metal heat-sink just waiting to condense all that water. You need a moisture tight barrier between you and the metal. Plastic,drop clothes,work.
you want a durable surface between you and the plastic... wood paneling, plastic political signs, anything but cardboard is good here. Between your vapor barrier and the van shell you want insulation. Polyiso or polystyrene is best R per inch and you don't have much room for,thick layers. i endorse 2 inches foamboard sealed with expanding foam as someone else said.
lastly, make sure the roof,of,your,van,is rain tight and painted and sealed before you install all this. Much harder to find and fix leaks after.